Recently, considering environmental issues, attention has been paid to a vehicle employing a motor as a drive power source, such as an electric vehicle, a hybrid vehicle and a fuel cell vehicle. Such a vehicle includes a power storage unit implemented, for example, by a rechargeable battery or an electric double layer capacitor for supplying electric power to the motor, and converting kinetic energy to electric energy during regenerative braking and storing such electric power.
In such a vehicle employing the motor as the drive power source, in order to enhance acceleration performance and running performance such as travel distance, greater charge/discharge capacity of the power storage unit is desired. Here, a configuration where a plurality of power storage units are mounted has been proposed as a method of increasing the charge/discharge capacity of the power storage unit.
For example, U.S. Pat. No. 6,608,396 discloses a power control system providing desired high DC voltage levels required by a high voltage vehicle traction system. The power control system includes a plurality of power stages for providing DC power to at least one inverter, each stage including a battery and boost/buck DC-DC converter, the power stages wired in parallel, and a controller controlling the plurality of power stages so as to maintain a voltage output to at least one inverter by causing uniform charge/discharge of the batteries of the plurality of power stages.
Meanwhile, as the power storage unit stores electric energy utilizing electrochemical process, the charge/discharge characteristic thereof is susceptible to a temperature. As the temperature is lower, the charge/discharge performance of a general power storage unit is lowered. Accordingly, in order to maintain prescribed charge/discharge performance, temperature management, particularly control of temperature increase, in the power storage unit is important.
For example, Japanese Patent Laying-Open No. 2003-272712 discloses a battery control device capable of preventing electric power that can be input/output to a battery from lowering due to temperature lowering, by efficiently raising a temperature of the battery while avoiding detriments caused by addition of parts. According to this battery control device, if the temperature of the battery is not higher than a prescribed value, charge/discharge of the battery is repeated in accordance with a state of charge of the battery.
Actual full charge capacity often differs between the power storage units even of the same type, depending on variation in manufacturing or degree of deterioration. Accordingly, as in the power control system disclosed in U.S. Pat. No. 6,608,396, even if uniform charge/discharge is carried out based on SOC of each battery, there is a difference in an amount of actually supplied electric charges (charged power). Consequently, a difference is created also in allowable power of the battery. Nevertheless, the power control system disclosed in U.S. Pat. No. 6,608,396 has not considered the allowable power of each battery. Therefore, power requirement in a load device (such as a motor) has not been met.
In addition, since the power control system disclosed in U.S. Pat. No. 6,608,396 is premised on the same fill charge capacity of the battery as designed, combination of a plurality of batteries different in the full charge capacity has not been permitted.
Meanwhile, in the battery control device disclosed in Japanese Patent Laying-Open No. 2003-272712, in order to raise the temperature of the battery, regenerative charging or drive of the generator is prohibited, depending on a state of charge of the battery. Therefore, running performance of the vehicle has been restricted by execution of control for temperature increase in the battery.